Spurious surface finish improvement of the infrared extinction coefficient of alpha quartz

Infrared extinction coefficients obtained using quartz samples lapped with a 3 mu m abrasive compound were found to be much smaller than anticipated. The improvement in extinction coefficient is traceable to the irregular surface finish. To demonstrate this improvement, polished quartz samples were evaluated and compared to evaluations after lapping the same sample with 3 mu m and 12 mu m abrasives. In one sample, a 3 mu m finish resulted in an almost 30% improvement in the extinction coefficient evaluated at 3500 cm/sup -1/. Lapping of the sample to a 12 mu m finish resulted in an almost 90% improvement. These improvements in alpha correspond to improvements in Q/sub IR/ (3500) from 2.58 million (polished) to 3.03 million (3 mu ) to 4.71 million (12 mu m). The improvement is caused by wavelength dependent transmission losses due to surface irregularities. The surface transmission losses can be reduced by polishing the optical surfaces or by applying index matching fluids to 3 mu m lapped surfaces.<>